1) One who engages in the practice of exploring the anus and rectal passage of another, just as a spelunker enjoys exploring dark, dank, underground passages.
2) A person who acts as though he might enjoy the practice of ass spelunking.

1) Man that guy is an ass spelunker. I think I saw him in the Gay Pride Parade last week.

an individual who explores the anal passageways of another, through use of a tool such as a dildo or a penis. derived from ass, the anal region of the human body, while spelunking is cave exploring, thus the act of exploring the cavities of ones asshole. also used to describe a homosexual.

An ass is somebody's bottom or more specifically in this case an anus.
A spelunker is an unexperienced person who has an interest in entering or enters caves.
Therefore, an Ass Spelunker is somebody who has an interest in exploring or does explore the cave-like orifice of an anus.

In South Park Terrance and Phillip call eachother Ass Spelunkers.
You love to fuck suzie in the butt, you ass spelunker!!!

Originally termed in the movie "South Park: Bigger, Longer, and Uncut." Commonly mis-spelled as ass beehonker, this term refers to anal intercourse between two men. Splunking itself is decending into a deep cave, which is fitting.